How do you increase a number *directly* under the cursor?

<C-a> and <C-x> work on whole numbers (1 or more digits) but not on “parts” of a number. However it takes a “count” so 10<C-a> turns [6]5 into 75.

Another option, since you only want to change one digit, would be to do r7 to turn [6]5 into 75.